The present invention relates to a gas sensing element used in a gas sensor installable in an exhaust gas passage of an internal combustion engine of an automotive vehicle for detecting a specific gas component, such as a NOx gas concentration, contained in the exhaust gas, or an air-fuel ratio sensor incorporated in an air-fuel ratio (A/F) control system of the internal combustion engine, or a sensor for detecting an oxygen gas concentration.
Automotive vehicles exhaust harmful emission gases, such as NOx, HC, CO, that cause serious air pollution. Nowadays, the global warming phenomenon caused by CO2 is a big social problem to be solved.
To prevent air pollution, law regulations have become severe and strict to substantial reduce of harmful emissions exhausted from automotive vehicles and also in detection of deteriorated catalytic converters which may not function to purify the exhaust gases.
To stop the global warming phenomenon, effective countermeasures will include reduction of CO.sub.2, restriction in fuel consumption, and favorable treatment in tax.
In the market of automotive vehicles, reduction of harmful emissions and improvement of fuel economy are main goals to be attained to respond to the requirements from the modern society.
To satisfy such requirements, lean burn engines including direct-injection type engines (injecting fuel directly in a combustion chamber) have been developed as prospective engines having the capability of improving the fuel economy of the gasoline engines.
The lean burn techniques are characterized in that the air-fuel ratio (A/F ratio) is set to be a higher level compared with a stoichiometric (or theoretical) value, i.e., 14.7, of the ordinary engines. In general, shifting of the air-fuel ratio to a higher level increases the NOx amount contained in the exhaust gas. The conventional three-way catalytic converters were chiefly developed to purify the stoichiometric exhaust gases. Thus, it is necessary to develop a new exhaust gas purification system effectively applicable to advanced lean burn techniques.
Meanwhile, there are advanced diesel engines that are electronically controlled. Reduction of NOx gas is also important for the diesel engines. In this respect, development of so-called DeNOx catalytic converter systems will be indispensable for the advanced diesel engines.
To develop the NOx catalyst based purification systems, it is important to accurately control the purification rate of the NOx catalyst or monitor the deterioration of the NOx catalyst. To realize this, it is desirable to directly detect the NOx gas concentration of the exhaust gas.
Unexamined Japanese patent publication No. 64-39545 (No. 1-39545) discloses a gas sensing element that is capable of directly detecting the NOx gas concentration of the exhaust gas. This conventional gas sensing element comprises two sets of cells, each consisting of an oxygen pump cell and a sensor cell. Each oxygen pump cell includes electrodes provided on opposite sides of a solid electrolytic member disposed between a sample gas chamber and an exhaust gas passage. Each sensor cell includes electrodes provided on opposite sides of a solid electrolytic member disposed between the sample gas chamber and a reference gas chamber. The exhaust gas is introduced via an introducing port into sample gas chamber. The NOx gas concentration is detectable by measuring an electric current value between the sensor cells.
FIGS. 9A and 9B are views showing another conventional gas sensing element (for example, disclosed in the Unexamined Japanese patent publication No. 8-271476). A gas sensing element 9 comprises two solid electrolytic members 901 and 902 between which a spacer is interposed. A sample gas chamber is formed in this spacer. The sample gas chamber consists of a first chamber 903 and a second chamber 904.
The sample gas is introduced via an introducing passage 905 into the first chamber 903. An oxygen sensor cell 91 detects the oxygen gas concentration in the first chamber 903. A drive voltage of a first oxygen pump cell 92 is feedback controlled so as to equalize a detected oxygen gas concentration with a predetermined value.
The oxygen sensor cell 91 includes two electrodes 911 and 912 provided on a surface of the solid electrolytic member 902. One electrode 911 is exposed to air in an air introducing passage 907, and the other electrode 912 is exposed to the gas in the first chamber 903. The first oxygen pump cell 92 comprises two electrodes 921 and 922 provided on opposite surfaces of the solid electrolytic member 901. One electrode 921 is exposed to the sample gas, and the other electrode 922 is exposed to the gas in the first chamber 903.
The second chamber 904 communicates with the first chamber 903 via a passage 906. A second oxygen pump cell 93 is provided in the second chamber 904 to discharge the oxygen gas from the second chamber 904. The second oxygen pump cell 93 comprises two electrodes 911 and 932 provided on the surface of the solid electrolytic member 902. The electrode 932, having NOx deoxidizing properties, is exposed to the gas in the second chamber 904.
In the second chamber 904, NOx contained in the exhaust gas is deoxidized and decomposed so as to newly generate oxygen gas. The pump current flowing through the second oxygen pump cell 93 increases or decreases in response to the generated oxygen gas.
The oxygen gas, contained in the sample gas diffusing from the first chamber 903 to the second chamber 904, has a constant concentration. From this fact, it is believed that the increase or decrease of the pump current is dependent on the deoxidization of NOx. In other words, the NOx gas concentration is detectable by measuring the pump current.
However, the introducing passage of the above-described gas sensing element is a pinhole which is usually formed by machining. The pinhole is formed by opening a through hole extending vertically across a sheet serving as a solid electrolytic member.
The gas amount diffusing through the pinhole is dependent on the ambient temperature T. Usually, when the pinhole is formed by machining, the gas diffusion amount is proportional to T.sup.1.75.
For this reason, the output current of the sensor element has temperature dependency. When the exhaust gas temperature changes in a wide range, a significant measuring error will be caused.
This is a common problem raised in gas sensor elements which are used for detecting the concentration of specific gas components, such as O.sub.2, HC and CO, involved in the sample gas.